nl80211: Fix enum type of variable in nl80211_put_sta_rate()

rate_flg is of type 'enum nl80211_attrs', however it is assigned with
'enum nl80211_rate_info' values. Change the type of rate_flg accordingly.

Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>